The term “lichen” is used to describe the symbiosis between a fungus and an alga. The fine fungal threads weave around the algae or penetrate them. This enables the exchange of substances between the two life partners. The alga uses sunlight and chlorophyll to produce starch, which is the basis of life for the fungus. Conversely, the fungus protects the alga from drying out and thus enables it to exist in otherwise inaccessible habitats. Both partners benefit from the partnership. Lichens are good indicators of harmful environmental influences.